Voltage regulators are commonly used in the power management systems of computers, mobile phones, automobiles and many other electronic products. Generally, voltage regulators are configured to convert an unstable power supply voltage into a stable one. A “low dropout” (LDO) regulator has a low input-to-output voltage difference between an input terminal to which an unstable power supply voltage is applied and the output terminal of the regulator which provides the stabilised voltage. Ideally, the dropout voltage should be as low as possible, to reduce the power consumption while still maintaining regulation performance.
FIG. 1 shows a circuit diagram of a known voltage regulator described in US 2013/0076325 A1. This known LDO regulator comprises a pass transistor 101, an operational amplifier 102, and a voltage divider circuit 103. The gate of the pass transistor is coupled to the operational amplifier's output which comprises a control signal which serves to regulate a supply voltage VIN which is applied to the source of the pass transistor. A regulated voltage VOUT thus appears at an output node 104 which is coupled to the drain of the pass transistor. The operational amplifier 102 has two inputs for receiving, respectively, a reference voltage VREF and a feedback voltage VFB (which is derived from VOUT), and generates the control signal according to a difference between the reference voltage VREF and the feedback voltage VFB. The voltage divider circuit 103 comprises a string of resistors and a stabilization element connected in series between the pass transistor's drain and ground. This voltage regulator comprises a closed loop topology. The operational amplifier drives the pass transistor with more current if, for example, VFB drops below VREF (owing to a variation in load current for example). Thus the voltage at VOUT is stabilised.
Voltage regulators may also, typically, be used to power ASICs. An ASIC (Application Specific Integrated Circuit) is a semiconductor device designed for a particular application and may include virtually any collection of known digital circuits. ASICs may be powered by one of the available various regulator technologies depending upon the needs of the circuit. For example, for applications requiring extremely low quiescent and active operating current but which can tolerate the use of an external (i.e., relatively large) capacitor, a linear (e.g., low dropout (LDO)) regulator is highly suitable. On the other and, if board space (or other physical space) is at a premium and higher quiescent and active operating currents are tolerable, then a “capless” regulator having no external capacitor may be a better solution. WO 2009/085439 describes an ASIC which includes both types of regulator which may be selectable by internal control circuitry. Both types of regulator are closed loop whose stability is affected by variations in load capacitance.
Digital circuits, such as those found in ASICs for example, often have load current profiles consisting of sharp and short current spikes. Such transient digital load currents can be typically several milliAmps although average current drain is usually small and of the order of a few microAmps. Decoupling capacitors are typically employed on the power supply line of a digital load in order to minimise transient voltage drops and the propagation of switching noise. However, the stability of a closed loop regulator depends on the total output capacitance. This may include any decoupling capacitors and the capacitance of the load. This is a disadvantage because of the difficulty in predicting both the amount of decoupling required and the load capacitance of the system to which the regulator will deliver the regulated voltage.